1. Field of the Invention
The invention relates to Quality of Service (QoS) in packet switched communication systems. Quality of Service is enforced by way of policy enforcement and control. Policy enforcement is applied at access network gateways under the control of a policy decision function. Particularly, the invention relates to a method for the transfer of information between policy decision functions during handover in a communication system. The information may be related, for example, to policy control.
2. Description of the Related Art
The transport of voice and multimedia over packet switched networks has in the recent years emerged as a viable alternative for traditional circuit switched networks. In circuit switched networks resource allocation is based on the allocation of an entire physical circuit or on the allocation of a repeating timeslot within a physical circuit for a given user. From an abstract point of view the transport technology relieves the network of complexity involving admission control and Quality of Service (QoS) allocation. In packet switched networks the transport technology inherently does not provide the users with guarantees involving the QoS available for a single user. QoS is observed in terms of such properties as, for example, data rate, delay, the variation of delay and bit error probability. These properties are usually referred to as QoS parameters. The users must be guaranteed certain QoS parameters. However, other users must also be taken into consideration before granting given QoS parameters for a given new user. In other words, it must be ensured that the capacity of the system is not exceeded when implementing the new users QoS requirements in the system. The QoS guarantees already committed to must be sustained. It must be checked that an increase in the use of a variety of resources such as packet queues in network nodes, network node packet switching capacity and transmission line capacity does not cause a relaxation from already guaranteed parameters such as maximum delays and data rates.
In order to control the access of new users to network resources admission control is applied. In packet switched networks admission control entities have been introduced to control access to network resources. The admission control entities are interfaced by users or by network nodes on behalf of users in order to perform QoS allocation for users. Admission control may be performed in small scale for individual users or for flows originated by individual users. In larger scale admission control may be performed for entire networks at the edge of a large core network by determining that the networks adhere to predefined service level agreements. Examples of technologies for the implementation of QoS in Internet Protocol (IP) networks include Integrated Services (IntServ) and Differentiated Services (DiffServ) defined in the Internet Engineering Task Force (IETF) documents RFC 2210 and RFC 2475, respectively. Yet another standard for the QoS is the Multi Label Protocol Switching (MPLS) defined in IETF document RFC 3032. In the Common Open Policy Services (COPS) framework defined in the IETF document RFC 2753, the admission control decisions are centralized to a Policy Decision Point (PDP), which makes decisions whether to admit a certain flow or set of flows to the network on behalf of a Policy Enforcement Point (PEP). The PEP is in practice a router or a network edge node. When receiving an allocation request for a flow, the PEP contacts the PDP. The PDP returns a policy decision to the PEP, which in effect tells whether the flow should be admitted or denied. The QoS parameters may entirely be provided from the PDP or simply modified by the PDP. The information regarding a flow or a set of flows is obtained to a PEP, for example, via the Resource Reservation Protocol (RSVP) defined in the IETF document RFC 2205.
In the Universal Mobile Telecommunications System (UMTS) the Common Open Policy Services protocol defined in the IETF document RFC 2748 or the Diameter protocol defined in the IETF document RFC 3588 is used to obtain QoS parameters for Packet Data Protocol (PDP) contexts based on at least one set of binding information provided from a Mobile Station (MS). Each such set of binding information may consist, for example, of an authorization token and a number of flow identifiers. The binding information may also consist of a user identifier or of a user equipment identifier, which may be accompanied by flow identifiers such as IP packet filters. The authorization token provides the Fully Qualified Domain Name (FQDN) of a Policy Decision Point (PDP) and a unique session identifier within the PDP. The flow identifiers identify uniquely either a single IP flow or a bi-directional combination of two IP flows associated with the session.
Reference is now made to FIG. 1, which illustrates a Universal Mobile Telecommunications System (UMTS) in prior art. In FIG. 1 there is shown a mobile station 100, which communicates with a Radio Network Controller (RNC) 114 within a Radio Access Network 110. The communication occurs via a Base Transceiver Station (BTS) 112. The radio access network 110 is, for example, a 2G GSM/EDGE radio access network or a 3G UMTS radio access network. An IP Connectivity Access Network (IP-CAN) functionality (not shown) connected to access network 110 comprises at least a Serving GPRS Support Node (SGSN) 122 and a Gateway GPRS Support Node (GGSN) 124. The functionality of a GPRS based IP-CAN is disclosed in the 3G Partnership Project specification 23.060. SGSN 122 performs all mobility management related tasks and communicates with a Home Subscriber Server (HSS) 160 in order to obtain subscriber information. GGSN 124 provides GPRS access points. There is an access point, for example, to a Media Gateway (MGW) 126, to a first router 142 attached to an IP network 140, and to a Proxy Call State Control Function (P-CSCF) 152. The access point to IP network is used to relay packets to/from an IP network node (IP-N) such as 147. The packets may be related to, for example, Internet browsing or File Transfer Protocol (FTP) file transfer. The access point for P-CSCF 152 is used to convey signaling traffic pertaining to IP multimedia. GGSN 124 establishes Packet Data Protocol (PDP) contexts, which are control records associated with a mobile subscriber such as mobile station 100. A PDP context provides an IP address for packets received from or sent to mobile station 100. A PDP context has also associated with it a UMTS bearer providing a certain QoS for mobile station 100. In GGSN 124 there is a primary PDP context for the signaling packets associated mobile station 100. For the user plane data packets carrying at least one IP flow there is established at least one secondary PDP context. The at least one IP flow is established between a calling terminal and a called terminal in association with an IP multimedia session. An IP flow carries a multimedia component such as a voice or a video stream in one direction. For voice calls at least two IP flows are required, one for the direction from the calling terminal to the called terminal and one for the reverse direction. In this case an IP flow is defined as a quintuple consisting of a source port, a source address, a destination address, a destination port and a protocol identifier.
The communication system illustrated in FIG. 1 comprises also the IP Multimedia Subsystem (IMS) functionality. The IMS is used to set-up multimedia sessions over IP-CAN. The network elements supporting IMS comprise at least one Proxy Call State Control Function (P-CSCF), at least one Inquiring Call State Control Function (I-CSCF), at least one Serving Call State Control Function S-CSCF, at least one Brakeout Gateway Control Function (BGCF) and at least one Media Gateway Control Function (MGCF). As part of the IMS there is also at least one Home Subscriber Server (HSS). Optionally, there is also at least one Application Server, which provides a variety of value-added services for mobile subscribers served by the IP multimedia subsystem (IMS). The IMS is disclosed in the 3G Partnership Project (3GPP) specification 23.228.
P-CSCF 152 receives signaling plane packets from GGSN 124. The P-CSCF usually comprises a Policy Decision Function (PDF), which corresponds to a Policy Decision Point (PDP) familiar from the COPS framework. The PDF may also be implemented as a separate PDP, which communicates with the P-CSCF. Without the authorization from the P-CSCF, a primary PDP context is opened in GGSN 124. Via the primary PDP context are sent signaling plane packets used to set-up an IP multimedia session between mobile station 100 and another a called party terminal (TE) 146. However, it should be noted that an un-guaranteed QoS IP multimedia session may be established with the called party terminal 146 or IP network node 147 via the access point connecting to router 142. The purpose of the IMS, among other things, is to provide a system for guaranteeing a certain QoS for the IP multimedia session. Session Initiation Protocol (SIP) signaling messages are carried in the signaling plane packets. The Session Initiation Protocol (SIP) is disclosed in the Internet Engineering Task Force (IETF) document RFC 3261. The signaling message is processed by P-CSCF 152, which determines the correct serving network for the mobile station 100 that sent the signaling packet. The determination of the correct serving network is based on a home domain name provided from mobile station 100. Based on the home domain name is determined the correct I-CSCF, which in FIG. 1 is I-CSCF 154. I-CSCF 154 hides the topology of the serving network from the networks, in which mobile station 100 happens to be roaming. I-CSCF 154 takes contact to home subscriber server 160, which returns the name of the S-CSCF, which is used to determine the address of S-CSCF 156 to which the mobile station 100 is to be registered. If I-CSCF 156 must select a new S-CSCF for mobile station 100, home subscriber server 160 returns required S-CSCF capabilities for S-CSCF selection. Upon receiving a registration, S-CSCF 156 obtains information pertaining to the profile of the mobile station 100 from HSS 160. The information returned from HSS 160 may be used to determine the required trigger information that is used as criterion for notifying an application server 162. Application server 162 may be notified on events relating to incoming registrations or incoming session initiations. Application server 162 communicates with S-CSCF 156 using the ISC-interface. The acronym ISC stands for IP multimedia subsystem Service Control interface. The ISC interface is disclosed in the 3GPP specification 23.228. The protocol used on ISC interface is SIP. AS 162 may alter SIP invite message contents that it receives from S-CSCF 156. The modified SIP invite message is returned back to S-CSCF 156.
If the session to be initiated is targeted to a PSTN subscriber or a circuit switched network subscriber, the SIP invite message is forwarded to a BGCF 158. BGCF 158 determines the network in which interworking to PSTN or the circuit switched network should be performed. In case PSTN interworking is to be performed in the current network, the SIP invite message is forwarded to MGCF 159 from BGCF 158. MGCF 159 communicates with MGW 126. The communication uses, for example, the MEGACO protocol defined in IETF document 3525. The user plane packets carrying a media bearer or a number of interrelated media bearers for the session are routed from GGSN 124 to MGW 126 as illustrated in FIG. 1.
If the session to be initiated is targeted to terminal 146, which is a pure IP terminal, S-CSCF 156 forwards the SIP Invite message to terminal 146. Terminal 146 communicates with a second router 144, which interfaces IP network 140. IP network 140 is used to carry the user plane IP flows associated with the session established between mobile station 100 and terminal 146. The user plane IP flows between first router 142 and GGSN 124 are illustrated with line 128. The user plane IP flows between second router 144 and terminal 146 are illustrated with line 148.
In order to allocate the end-to-end QoS required for the user plane IP flows between mobile station 100 and terminal 146, the GGSN 124 provides to a PDF within P-CSCF 152 at least one set of binding information provided from a mobile station 100. If token based binding is used, the sets of binding information have been formed in the PDF within P-CSCF 152 in response to SIP signaling and the Session Description Protocol (SDP) definitions carried in the SIP signaling messages. In order to form a set of binding information, the PDF has allocated a unique identifier for a session to be established and has assigned unique flow identifiers for each IP flow or each bi-directional combination of two IP flows observed in the SDP definitions. The unique identifier together with the PDF FQDN is used to form an authorization token for the session in the PDF. The authorization token is returned to mobile station 100 as binding information. There may be other authorization tokens for other parallel sessions. Mobile station 100 also assigns unique flow identifiers for each IP flow or each bi-directional combination of two IP flows observed in the SDP definitions in the same way as the PDF. Instead of a token, the binding may be based on other mechanisms, for example, on user identification or user equipment identification and at least one flow filter.
The mobile station 100 sends the binding information, for example, the authorization token and the flow identifiers of the IP flows or bi-directional IP flow combinations to be set up, to the GGSN 124 upon the secondary PDP context establishment. The GGSN 124 sends the binding information to the PDF in an authorization request. In response to the sets of binding information, the PDF returns the QoS information for the IP flows identified in the sets of binding information. The QoS information is used to establish a UMTS bearer between GGSN 124 and mobile station 100. The QoS information is also used to establish an external bearer between GGSN 124 and terminal 146. The UMTS bearer is established using signaling towards SGSN 122 and from there to RNC 114. The UMTS bearer comprises a radio access bearer and a core network bearer. The external bearer is established from GGSN 124 either explicitly using RSVP signaling or implicitly by assigning the user plane packets associated with an IP flow a certain Differentiated Service Code Point (DSCP).
Reference is now made to FIG. 2, which illustrates a dual-system mobile station and two different IP connectivity access networks connected to a single IP multimedia subsystem in prior art. In FIG. 2 there is a communication system 200 comprising an IP Multimedia Subsystem (IMS) 250, two IP Connectivity Access Networks (IP-CAN), namely IP-CAN 210 and IP-CAN 220, and a mobile station 202. Mobile station 202 may also support fixed network access, in other words, it may be connected via a cable or a short range wireless interface to a fixed access network. IP-CAN 210 and IP-CAN 220 may represent different access network technologies, fixed or wireless, such as, for example, Digital Subscriber Line (xDSL), Worldwide Interoperability for Microwave Access (WiMAX), IEEE 802.11b or IEEE 802.11g Wireless Local Area Network (WLAN), GSM or UMTS. IP-CAN 210 and IP-CAN 220 may also represent networks based on same technologies, but may be administered by different network operators. IP-CAN 210 communicates with IMS 250 via gateway (GW) 212. IP-CAN 220 communicates with IMS 250 via gateway (GW) 222. The gateways may be, for example, GPRS GGSNs or gateway nodes for other types of access networks. Generally, a gateway node performs such tasks as, for example, providing at least one bearer for communicating with mobile station 202, QoS policy enforcement and packet marking, and network address translation. The gateways 212 and 222 communicate with P-CSCFs 240 and 242, respectively. The gateways 212 and 222 comprise policy enforcement functions, which obtain policy information from Policy Decision Functions (PDF) 230 and 232, respectively. The PDFs 230 and 232 may be comprised in P-CSCFs 240 and 242, respectively, or each of the PDFs may be a standalone entity or be integrated in a gateway. A single P-CSCF may communicate with a number of PDFs and vice versa.
Whenever required, gateways 212 and 222 establish an external bearer towards a media gateway (MGW) 262, which interfaces a circuit switched network such as PSTN 280. The gateways 212 and 222 may also establish external bearers directly to gateways or Session Border Controllers (SBC) in other IP-CANs or directly to end-user terminals. An external bearer should be distinguished from an internal bearer, which connects a gateway to an end-user station in an IP-CAN. An external bearer may carry a single multimedia component or a number of multimedia components. In FIG. 2 the external bearers from gateways 212 and 222 to MGW 262 are illustrated with lines M1 and M2. In FIG. 2 there is also an S-CSCF 254, I-CSCFs 270 and 272, which communicate with P-CSCFs 240 and 242, respectively, and determine using an HSS 252 the S-CSCF, which currently serves mobile station 202, for example S-CSCF 254. In FIG. 2 there are also illustrated an AS 256, BGCF 258 and MGCF 260.
The problem in prior art solutions such as illustrated in FIGS. 1 and 2 is that it currently not possible to change the P-CSCF during an ongoing session for an end-user station. If a new P-CSCF is to be allocated for the session due to the use of a new IP-CAN, the new P-CSCF does not obtain the session related parameters comprising information on the multimedia components and their QoS requirements from the old P-CSCF to be provided to the new PDF under the control of the new P-CSCF. Furthermore, currently a mobile station does not take heed on the changing of the P-CSCF during a handover. Thus, a new mechanism is needed, which supports the changing of a P-CSCF during an ongoing session and provides the new P-CSCF with the session related parameters necessary to perform policy decisions at the establishment of bearers from the end-user station. The problems associated with the lack of session related parameters may lead to the obtaining of extra bandwidth during handovers since the gateway does not get information on the QoS parameters to be applied for the bearers of the end-user station that performed the handover.